Oxide for semiconductor layer of thin-film transistor, sputtering target, and thin-film transistor

ABSTRACT

This oxide for a semiconductor layer of a thin-film transistor contains Zn, Sn and In, and the content (at %) of the metal elements contained in the oxide satisfies formulas (1) to (3) when denoted as [Zn], [Sn] and [In], respectively. [In]/([In]+[Zn]+[Sn])≧−0.53×[Zn]/([Zn]+[Sn])+0.36 (1) [In]/([In]+[Zn]+[Sn])≧2.28×[Zn]/([Zn]+[Sn])−2.01 (2) [In]/([In]+[Zn]+[Sn])≦1.1×[Zn]/([Zn]+[Sn])−0.32 (3) The present invention enables a thin-film transistor oxide that achieves high mobility and has excellent stress resistance (negligible threshold voltage shift before and after applying stress) to be provided.

TECHNICAL FIELD

The present invention relates to an oxide for a semiconductor layer of athin-film transistor to be used display devices such as a liquid crystaldisplay, an organic EL display; a sputtering target for forming a filmof the oxide; and a thin-film transistor.

BACKGROUND ART

As compared with widely used amorphous silicon (a-Si), an amorphous(noncrystalline) oxide semiconductor has high carrier mobility, a highoptical band gap, and film formability at low temperature and,therefore, has been highly expected to be applied for next generationdisplays which are required to have a large size, high resolution, andhigh-speed drive, resin substrates which has low heat resistance, andthe like.

Of oxide semiconductors, an amorphous oxide semiconductor containingindium, gallium, zinc, and oxygen (In—Ga—Zn—O, hereinafter also referredto as “IGZO”), which has a considerably high carrier mobility, isparticularly preferably used. For example, Non-Patent Documents 1 and 2disclose a thin-film transistor (TFT) including a thin oxidesemiconductor film of In:Ga:Zn=1.1:1.1:0.9 (atomic % ratio) as asemiconductor layer (active layer). Patent Document 1 also discloses anamorphous oxide containing elements, such as In, Zn, Sn, Ga, and thelike, and Mo, where Mo has an atomic composition ratio of 0.1 to 5atomic % with respect to the total number of metal atoms in theamorphous oxide. A TFT including an active layer of IGZO doped with Mois disclosed in the example of Patent Document 1.

PRIOR ART DOCUMENT Patent Document

-   Patent Document 1: JP-A-2009-164393

Non-Patent Document

-   Non-patent Document 1: solid physics, vol44, P621 (2009)-   Non-patent Document 2: Nature, vol432, P488 (2004)

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In the case where an oxide semiconductor is used as a semiconductorlayer for a thin-film transistor, the oxide semiconductor is requirednot only to have a high carrier concentration but also to be excellentin switching properties (transistor characteristics) of TFT.Specifically, the oxide semiconductor is required to satisfy (1) highON-current (maximum drain current when positive voltage is applied to agate electrode and a drain electrode); (2) low OFF-current (draincurrent when negative voltage is applied to a gate electrode andpositive voltage is applied to a drain electrode); (3) low SS value(Subthreshold Swing, gate voltage required to increase drain current byone digit); (4) stability of threshold with the lapse of time (voltageat which drain current starts flowing when positive voltage is appliedto a drain electrode and either positive or negative voltage is appliedto a gate voltage, which is also referred to as threshold voltage) (itmeans uniform even in in-place of substrate); (5) a high mobility; (6) asmall change in above mentioned properties at the time of lightirradiation, and the like. The inventors of the present invention haveinvestigated the above properties of ZTO containing Mo semiconductordescribed in previously mentioned Patent Document 1. As a result, theyhave found that it showed degradation of ON-current and elevation of SSvalue compared with that of ZTO.

Furthermore, a TFT using an oxide semiconductor layer of IGZO and ZTO,and the like are required to be excellent in resistance (stressstability) to voltage application and stress of light irradiation, andthe like. For example, when positive voltage or negative voltage iscontinuously applied to gate voltage or when light in a blue emittingband in which light absorption starts is continuously irradiated, thethreshold voltage is changed (shifted) considerably, and it is pointedout that because of that, the switching properties of the TFT arechanged. And for example, at the time of driving a liquid crystal panelor at the time of lighting a pixel by applying negative bias to a gateelectrode, and the like, the TFT is irradiated with light leaked outfrom a liquid crystal cell and this light gives stress to the TFT tocause deterioration of the properties such as elevation of theOFF-current, shift of the threshold voltage, and increase of the SSvalue, and the like. Particularly, shift of the threshold voltage leadsto lowering of reliability in a display device itself such as a liquidcrystal display or an organic EL display equipped with TFT, and,therefore, it has been desired to improve the stress stability (smallchange before and after stress tests).

The present invention has been made in view of the above situation. Itis an object of the present invention to provide an oxide for athin-film transistor, which has a high mobility and excellent stressstability (a small threshold voltage shift between before and afterstress tests), a thin-film transistor including the oxide, and asputtering target for use in forming the oxide.

Means for Solving the Problems

An oxide for semiconductor layer of a thin-film transistor of thepresent invention which can be solved above problems is a oxide to beused for the semiconductor layer of the transistor, wherein the oxidecontains Zn, Sn and In, and in the case where the content (atomic %) ofmetal elements contained in the oxide is defined as [Zn], [Sn], and [In]respectively, the content satisfies below expressions (1) to (3).

[In]/([In]+[Zn]+[Sn])≧−0.53×[Zn]/([Zn]+[Sn])+0.36  (1)

[In]/([In]+[Zn]+[Sn])≧2.28×[Zn]/([Zn]+[Sn])−2.01  (2)

[In]/([In]+[Zn]+[Sn])≦1.1×[Zn]/([Zn]+[Sn])−0.32  (3)

The above oxide preferably satisfies below expression (6)

[In]/([In]+[Zn]+[Sn])≧0.05  (6)

A thin-film transistor having the above oxide as a semiconductor layerof the thin-film transistor is included in the present invention.

The density of the above semiconductor layer is preferably 5.8 g/cm³ orhigher.

Also, a sputtering target of the present invention is a sputteringtarget for forming the above oxide, wherein in the case where thecontent (atomic %) of metal elements contained in the suputtering targetis defined as [Zn], [Sn] and [In] respectively, the content satisfiesbelow expressions (1) to (3).

[In]/([In]+[Zn]+[Sn])≧−0.53×[Zn]/([Zn]+[Sn])+0.36  (1)

[In]/([In]+[Zn]+[Sn])≧2.28×[Zn]/([Zn]+[Sn])−2.01  (2)

[In]/([In]+[Zn]+[Sn])≦1.1×[Zn]/([Zn]+[Sn])−0.32  (3)

The above sputtering target preferably satisfies below expression (6)

[In]/([In]+[Zn]+[Sn])≧0.05  (6)

Effects of the Invention

With the oxide of the present invention, a thin-film transistor having ahigh mobility and excellent stress stability (a smaller thresholdvoltage shift between before and after stress tests) can be provided. Asa result, a display device, which includes the thin-film transistor, hasa greatly improved level of reliability against light irradiation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view for illustrating a thin-filmtransistor of the present invention having a present inventive oxidesemiconductor.

FIG. 2 is a graph showing a region, which satisfies a range ofexpressions (1) to (3) defined in the present invention.

FIG. 3 is a view showing TFT characteristics before and after stresstests in the case of a portion data of the Examples.

MODE FOR CARRYING OUT THE INVENTION

The present inventors have extensively studied an oxide containing Zn,Sn, and In (hereinafter also represented by “IZTO”) in order to improveTFT characteristics and stress stability of a TFT including an activelayer (semiconductor layer) which is formed of that oxide. As a result,the present inventors have found that the desired object is achieved byusing, as a semiconductor layer of a TFT, an IZTO whose compositionratio of the metal elements is appropriately controlled. The presentinvention has been made based on this finding. The oxide of the presentinvention allows for a TFT having excellent TFT characteristics(specifically, a high mobility, a high ON-current, a low SS value, and asmall absolute value of a threshold voltage (Vth) in the vicinity of 0V)and a small change in transistor characteristics between before andafter stress tests (specifically, a smaller change rate (ΔVth) of Vthafter stress tests (light irradiation and negative bias)).

Specifically, the present inventors have conducted numerous preliminaryexperiments on the influence of In, Zn, and Sn on the TFTcharacteristics and the stress stability to find the followings: (a)although In contributes to an improvement in the mobility, a largeamount of In added leads to a decrease in the stability (resistance)against light stress or is likely to make the TFT conductive; (b)although Zn improves the stability against light stress, a large amountof Zn added leads to a sharp decrease in the mobility or a decrease inthe TFT characteristics or the stress stability; and (c) although,similar to Zn, Sn effectively improves the stability against lightstress, and the addition of Sn hinders or prevents the IZTO frombecoming conductive, a large amount of Sn added leads to a decrease inthe mobility or a decrease in the TFT characteristics or the stressstability.

Based on these findings, the present inventors have further studied andfound the following. That is, in the case where the content (atomic %)of metal elements contained in an oxide is defied as [Zn], [Sn], and[In] respectively, the ratio of [In] represented by[In]/([In]+[Zn]+[Sn]) (hereinafter also simply abbreviated to “Inratio”) in relation to the ratio of [Zn] represented by [Zn]/([Zn]+[Sn])(hereinafter also simply abbreviated to “Zn ratio”) satisfies allexpressions (1) to (3) below, the oxide has satisfactorycharacteristics. The present invention has been made based on thefinding.

[In]/([In]+[Zn]+[Sn])≧−0.53×[Zn]/([Zn]+[Sn])+0.36  (1)

[In]/([In]+[Zn]+[Sn])≧2.28×[Zn]/([Zn]+[Sn])−2.01  (2)

[In]/([In]+[Zn]+[Sn])≦1.1×[Zn]/([Zn]+[Sn])−0.32  (3)

FIG. 2 shows regions of above expressions (1) to (3). A hatched portionof FIG. 2 indicates a region in which all expressions (1) to (3) aboveare satisfied. In FIG. 2, characteristics results of examples describedbelow are also plotted. Some results that fall within the hatched rangeof FIG. 2 are satisfactory in terms of all of saturated mobility, TFTcharacteristics, and stress stability (circles in FIG. 2), while theother results that do not fall within the hatched portion of FIG. 2(i.e., the results do not satisfy all of above expressions (1) to (3))have at least one of the above properties that is reduced (crosses inFIG. 2).

Of above expressions (1) to (3), expressions (1) and (2), which mainlyrelate to the mobility, have been prepared based on numerous preliminaryexperiments to define the In ratio for achieving a high mobility inassociation with the Zn ratio.

Expression (3), which mainly relates to an improvement in the stressstability and the TFT characteristics (TFT stability), has been preparedbased on numerous preliminary experiments to define the In ratio forachieving high light stress stability in association with the Zn ratio.

Specifically, it has been found that most of the results which do notsatisfy all of expressions (1) to (3) have drawbacks described below.

Firstly, IZTOs which satisfy expression (2) and do not satisfyexpressions (1) and (3) have larger Sn ratios (i.e., smaller Zn ratios).Therefore, for such IZTOs, although the mobility tends to increase, theS value or the Vth value tends to increase, and therefore, the TFTcharacteristics and the stress stability tend to decrease. Therefore,the desired characteristics are not obtained (see, for example, No. 13in the examples below).

Also, IZTOs which satisfy expressions (1) and (3) and do not satisfyexpression (2) have larger Zn ratios (i.e., smaller Sn ratios).Therefore, for such IZTOs, the mobility tends to sharply decrease, andthe S value or the Vth value tends to significantly increase, andtherefore, the TFT characteristics and the stress stability tend todecrease. Therefore, the desired characteristics are not obtained (see,for example, No. 14 in the examples below).

On the other hand, for IZTOs which satisfy expressions (1) and (2) anddo not satisfy expression (3) and which have larger In ratios, themobility tends to increase, and the stress stability tends to decrease.Therefore, the desired characteristics are not obtained (see, forexample, Nos. 9 and 10 in the examples below).

The oxide for the TFT semiconductor layer according to the presentinvention satisfies the above requirements. Furthermore, the ratio of[In] to ([Zn]+[Sn]+[In]) is preferably 0.05 or higher. Specifically, theratio of [In] preferably satisfies following expression (6):

[In]/([In]+[Zn]+[Sn])≧0.05  (6)

As described above, In is an element that increases the mobility. Whenthe ratio of [In] represented by expression (6) is less than 0.05, theabove effect is not effectively exhibited. More preferably, the In ratiois 0.1 or higher. On the other hand, if the In ratio is excessivelyhigh, the stress stability decreases or the oxide is likely to becomeconductive. Therefore, mostly, the In ratio is preferably 0.5 or less.

The oxide of the present invention is described above.

The above oxide is preferably formed in a film using a sputtering target(which may be hereinafter referred to as a “target”) with a sputteringmethod. The oxide can be formed by a chemical film formation method suchas a coating method; however, it is possible to easily form a thin filmexcellent in film in-plane uniformity of components and film thicknessaccording to the sputtering method.

As a target to be used in the sputtering method, there may preferably beused a target containing the elements described above and having thesame composition as that of a desired oxide, thereby making it possibleto form a thin film having a desired component composition without apossibility of a composition gap. More specifically, the content (atomic%) of metal elements contained in the sputtering target defined as [Zn],[Sn] and [In] respectively satisfies following expressions (1) to (3):

[In]/([In]+[Zn]+[Sn])≧−0.53×[Zn]/([Zn]+[Sn])+0.36  (1)

[In]/([In]+[Zn]+[Sn])≧2.28×[Zn]/([Zn]+[Sn])−2.01  (2)

[In]/([In]+[Zn]+[Sn])≦1.1×[Zn]/([Zn]+[Sn])−0.32  (3)

Above sputtering target preferably satisfies following expression (6):

[In]/([In]+[Zn]+[Sn])≧0.05  (6)

Alternatively, film formation may be carried out by a co-sputteringmethod for simultaneously discharging two targets with differentcompositions and consequently, a film with a desired composition can beobtained by co-sputtering targets such as In₂O₃, ZnO, SnO₂, and the likeor a target of mixture thereof.

The above-mentioned targets can be produced by, for example, a powdersintering process method.

In the case of sputtering the above-mentioned target, it is preferablethat the substrate temperature is adjusted to room temperature and theconcentration of oxygen is controlled properly for the execution. Theconcentration of oxygen may be controlled properly in accordance withthe configuration of a sputtering apparatus and the target composition,and the like, and it is preferable to add oxygen in such a manner thatthe carrier concentration of the oxide semiconductor is approximately10¹⁵ to 10¹⁶ cm⁻³. The concentration of oxygen in examples of theinvention is controlled such that it satisfies O₂/(Ar+O₂)=2% in additionflow ratio.

Also, in the case where the above-mentioned oxide is used as thesemiconductor layer of the TFT, the density of the oxide semiconductorlayer is preferably 5.8 g/cm³ or higher (described below), and in orderto form a film of such an oxide, it is preferable to properly controlthe gas pressure during sputtering, the power input to a sputteringtarget, the substrate temperature, and the like. For example, if the gaspressure is made low at the time of film formation, scattering ofsputtered atoms one another can be prevented and it is supposed to bepossible to form a compact (highly dense) film, and due to that, it isgood as the entire gas pressure at the time of film formation is low toan extent that the discharge for sputtering is stabilized, and thepressure may be controlled preferably in a range of approximately 0.5 to5 mTorr and more preferably in a range of 1 to 3 mTorr. Also, it is goodas the power input is high, and it is recommended to set the power inputof DC or RF to approximately 2.0 W/cm² or higher. It is also good as thesubstrate temperature at the time of film formation is high, and it isrecommended to set the temperature to a range around room temperature to200° C.

The film thickness of the oxide formed into a film as described above ispreferably 30 nm or more and 200 nm or less, and more preferably 35 nmor more and 80 nm or less.

The present invention also encompasses a TFT having the above-mentionedoxide as a semiconductor layer of the TFT. The TFT may have at least agate electrode, a gate insulator layer, a semiconductor layer of theabove-mentioned oxide, a source electrode, and a drain electrode on asubstrate, and its configuration is not particularly limited as long asit is used commonly.

Herein, the density of the above oxide semiconductor layer is preferably5.8 g/cm³ or higher. If the density of the oxide semiconductor layer ishigh, defects in the film are decreased to improve the film quality, andalso since the interatomic distance is narrowed, the electronfield-effect mobility of a TFT element is significantly increased andthe electric conductivity becomes high and the stability to stress bylight irradiation is improved. The density of the above oxidesemiconductor layer is good as it is higher, and it is more preferably5.9 g/cm³ or more and further preferably 6.0 g/cm³ or more. The densityof the oxide semiconductor layer is measured by a method described inexamples below.

As shown in the examples below, as the density of the oxidesemiconductor layer increases, the amount of a threshold voltage change(ΔVth) after the stress test (light irradiation and negative biasapplication) tends to decrease, so that the stability against stress andthe reliability of the display device are improved. In other words, asthe density of the oxide semiconductor layer increases, the region withsatisfactory stability against stress increases. Therefore, thecomposition of the oxide semiconductor layer which is preferable inorder to obtain a preferable ΔVth may vary depending on the density ofthe oxide semiconductor layer.

Specifically, ΔVth after the stress test, which was measured by atechnique described in the examples below, is evaluated and classifiedas follows: “satisfactory (circles)” when the absolute value of ΔVth is15 V or less; specifically, “more satisfactory (double circles)” whenthe absolute value of ΔVth is 10 V or less; and more specifically, “evenmore satisfactory (stars)” when the absolute value of ΔVth is 6 V orless. In this case, composition ranges of the oxide semiconductor layerwhich are preferable in order to obtain the above regions (indicated bycircles, double circles, and stars) and depend on the density of theoxide semiconductor layer, can be represented by expressions describedbelow (below expressions (3) to (5)). The regions represented byexpressions (4) and (5) below are also shown in FIG. 2.

As can be seen from FIG. 2, the lines of expressions (3) to (5) have thesame slope and different intercepts. The acceptable range of the ratioof [In] is strictly limited in ascending order of expression No. (i.e.,(3)→(4)→(5)).

(a) When the density of the oxide semiconductor layer is 5.8 g/cm³ orhigher and smaller than 5.9 g/cm³:

in order to obtain the composition of the oxide semiconductor layerwhose ΔVth has an absolute value of 15 V or less (“circle”), it ispreferable to satisfy expression (3); in order to obtain the compositionof the oxide semiconductor layer whose ΔVth has an absolute value of 10V or less (“double circle”), it is insufficient to only satisfyexpression (3), and it is preferable to satisfy expression (4); and inorder to obtain the composition of the oxide semiconductor layer whoseΔVth has an absolute value of 6 V or less (“star”), it is insufficientto only satisfy expression (4), and it is preferable to satisfyexpression (5).

[In]/([In]+[Zn]+[Sn])≦1.1×[Zn]/([Zn]+[Sn])−0.32  (3)

[In]/([In]+[Zn]+[Sn])≦1.1×[Zn]/([Zn]+[Sn])−0.52  (4)

[In]/([In]+[Zn]+[Sn])≦1.1×[Zn]/([Zn]+[Sn])−0.68  (5)

For example, Nos. 6 and 9 in Table 2 described below indicate the oxidesemiconductor layers having a density of 5.8 g/cm³ which satisfyexpression (3), and therefore, have an absolute value of ΔVth of 13.4 V(No. 6) and 10.3 V (No. 9), which meet the pass criterion for “circle.”In contrast to this, No. 3 in Table 2 indicates the oxide semiconductorlayer having a density of 5.8 g/cm³ which satisfy not only expression(3) but also expression (4), and therefore, has an absolute value ofΔVth of 7.4 V, which meets the pass criterion for “double circle.”

(b) When the density of the oxide semiconductor layer is 5.9 g/cm³ orhigher and smaller than 6.0 g/cm³:

In order to obtain the composition of the oxide semiconductor layerwhose ΔVth has an absolute value of 15 V or less (“circle”) or 10 V orless (“double circle”), it is in both cases preferable to satisfyexpression (3); and in order to obtain the composition of the oxidesemiconductor layer whose ΔVth has an absolute value of 6 V or less(“star”), it is preferable to satisfy expression (4). Specifically, whenthe density of the oxide semiconductor layer is higher than that of (a),the composition range (acceptable range) of the oxide semiconductorlayer which is preferable in order to obtain the same ΔVth (evaluationreference) becomes wider, and therefore, even if expression (5) is notsatisfied, then when expression (4) is only satisfied, an absolute valueof ΔVth of 6 V or less (“star”) is obtained.

For example, No. 5 in Table 2 below indicates the oxide semiconductorlayer having a density of 5.9 g/cm³ which satisfies expression (3), andtherefore, has an absolute value of ΔVth of 10.7, which meets the passcriterion for “circle.” Similarly, No. 8 in Table 2 indicates the oxidesemiconductor layer having a density of 5.9 g/cm³ which satisfies notonly expression (3) but also expression (4), and therefore, has anabsolute value of ΔVth of 6.9 V, which meets the pass criterion for“double circle.”

(c) When the density of the oxide semiconductor layer is 6.0 g/cm³ orhigher:

in order to obtain the composition of the oxide semiconductor layerwhose ΔVth has an absolute value of 15 V or less (“circle”), 10 V orless (“double circle”), or 6 V or less (“star”), it is in all casespreferable to satisfy expression (3). Specifically, when the density ofthe oxide semiconductor layer is higher than that of (b), thecomposition range (acceptable range) of the oxide semiconductor layerwhich is preferable in order to obtain the same ΔVth (evaluationreference) becomes wider, and therefore, even if expression (4) is notsatisfied, then when expression (3) is only satisfied, an absolute valueof ΔVth of 6 V or less (“star”) is obtained.

For example, Nos. 1, 4, and 7 in Table 2 below indicate the oxidesemiconductor layers having a density of 6.2 g/cm³ which satisfyexpression (3), and therefore, have an absolute value of ΔVth of 1.3 V(No. 1), 6.0 V (No. 4), and 4.2 V (No. 7), which meet the pass criterionfor “star.”

Hereinafter, by referring to FIG. 1, embodiments of a method forproducing the above-mentioned TFT will be described. FIG. 1 and thefollowing production method describe one example of preferredembodiments of the present invention, and it is not intended that thepresent invention be limited thereto. For example, FIG. 1 shows a TFTwith a bottom gate type structure, however, the TFT is not limitedthereto, and the TFT may be a top gate type TFT having a gate insulatorlayer and a gate electrode successively on an oxide semiconductor layer.

As shown in FIG. 1, a gate electrode 2 and a gate insulator layer 3 areformed on a substrate 1 and an oxide semiconductor layer 4 is formedfurther thereon. A source-drain electrode 5 is formed on the oxidesemiconductor layer 4 and a passivation layer (insulator layer) 6 isformed thereon and a transparent conductive film 8 is electricallyconnected to the drain electrode 5 through a contact hole 7.

A method for forming the gate electrode 2 and the gate insulator layer 3on the substrate 1 is not particularly limited and methods commonly usedcan be adopted. Also, the kinds of the gate electrode, and the gateinsulator layer 3 are not also particularly limited and widely used onescan be used. For example, metals such as Al and Cu with low electricresistance and their alloys can be preferably used for the gateelectrode. Also, typical examples of the gate insulator layer include asilicon oxide film, a silicon nitride film, and a silicon oxynitridefilm, and the like. Additionally, metal oxides such as TiO₂, Al₂O₃ andY₂O₃ and those formed by layering them can be also used.

Next, the oxide semiconductor layer 4 is formed. The oxide semiconductorlayer 4 is preferable to be formed into a film by, as described above, aDC sputtering method or an RF sputtering method using a sputteringtarget with the same composition as that of the thin film.Alternatively, the film formation may be carried out by a co-sputteringmethod.

After wet etching, the oxide semiconductor layer 4 is subjected topatterning. It is preferable to carry out heat treatment (pre-annealing)for improving the film quality of the oxide semiconductor layer 4immediately after the patterning and accordingly, the ON-current andelectron field-effect mobility, which are transistor characteristics,are increased and the transistor performance is improved. The preferablepre-annealing condition is, for example, temperature: about 250 to 350°C., time: about 15 to 120 minutes.

After pre-annealing, the source-drain electrode 5 is formed. The kind ofthe source-drain electrode 5 is not particularly limited and widely usedones can be used. For example, similarly to the gate electrode, metalssuch as Al and Cu and their alloys may be used, or pure Ti as describedin examples below may be used, or further laminated structure of metalsand the like may be used.

A method for forming the source-drain electrode 5 may be carried out by,for example, forming a metal thin film by a magnetron sputtering methodand forming the metal thin film into the source-drain electrode 5 by alift-off method. Alternatively, there is a method for forming thesource-drain electrode 5 by previously forming a prescribed metal thinfilm by a sputtering method and thereafter forming the electrode bypatterning, not forming the electrode by the lift-off method asdescribed above; however, this method deteriorates the transistorcharacteristics since the oxide semiconductor layer is damaged at thetime of etching of the electrode. Therefore, in order to avoid suchproblems, a method including previously forming a passivation layer onthe oxide semiconductor layer, and subsequently forming the electrode bypatterning is adopted, and this method is used in examples describedbelow.

Next, the passivation layer (insulator layer) 6 is formed on the oxidesemiconductor layer 4 by a CVD (Chemical Vapor Deposition) method. Thesurface of the oxide semiconductor layer 4 is converted easily to beconductive by plasma-induced damage due to CVD (it is supposedlyattributed to that oxygen deficiency formed on the surface of the oxidesemiconductor becomes an electron donor), and in order to avoid theproblems, N₂O plasma irradiation is carried out before film formation ofthe passivation layer in examples described below. The conditiondescribed in the following document is adopted as the N₂O plasmairradiation condition.

J. Park, et. al, Appl. Phys. Lett., 93, 053505 (2008).

Next, according to a common method, the transparent conductive film 8 iselectrically connected to the drain electrode 5 through the contact hole7. The kinds of the transparent conductive film and drain electrode arenot particularly limited, and those which are used commonly can be used.As the drain electrode, materials exemplified for the above-mentionedsource-drain electrodes can be used.

EXAMPLES

Below, by way of examples, the present invention will be morespecifically described. However, the present invention is not limited bythe following examples. It is naturally understood that modificationsmay be properly made and practiced within the scope adaptable to themeaning described above and below. All of these are included in thetechnical scope of the present invention.

Example 1

According to the above-mentioned method, a thin-film transistor (TFT)shown in FIG. 1 was produced and the TFT characteristics and the stressstability were evaluated.

First, a Ti thin film with a thickness of 100 nm as a gate electrode anda gate insulator layer SiO₂ (200 nm) were successively formed on a glasssubstrate (EAGLE 2000 manufactured by Corning Incorporated, diameter 100mm×thickness 0.7 mm). The gate electrode was formed by using a pure Tisputtering target by a DC sputtering method in conditions as follows:film formation temperature: room temperature, film formation power: 300W, carrier gas: Ar, and gas pressure: 2 mTorr. Also, the gate insulatorlayer was formed by a plasma CVD method in conditions as follows:carrier gas: mixed gas of SiH₄ and N₂O, film formation power: 100 W, andfilm formation temperature: 300° C.

Next, oxide (IZTO) thin films with various compositions as described inTable 1 were formed by a sputtering method using sputtering targets(described below). For comparison, ZTO (conventional example) containingno In was also formed. An apparatus used for the sputtering was “CS-200”manufactured by ULVAC, Inc. and the sputtering conditions were asfollows.

Substrate temperature: room temperature

Gas pressure: 5 mTorr

Oxygen partial pressure: O₂/(Ar+O₂)=2%

Film thickness: 50 nm

Size of target used: φ4 inch×5 mm

Input power (DC): 2.55 W/cm²

The IZTO films having different compositions were formed by RFsputtering method using two sputtering targets having differentcompositions (a sputtering target of In₂O₃ and a sputtering targethaving different ZnO and Zn/Sn ratios), or by RF sputtering method usinga single IZTO sputtering target having the same composition as that ofthe desired oxide. Also, a ZTO film (conventional example) was formed byco-sputtering method in which electric discharge is simultaneouslyapplied to an oxide target (Zn—Sn—O) having a ratio of Zn:Sn of 6:4(atomic percent ratio) and an oxide target of ZnO.

The contents of the respective metal elements in the oxide thin filmsobtained in this manner were analyzed by XPS (X-ray PhotoelectronSpectroscopy) method.

After the thin oxide films were thus formed as above, patterning wasperformed by photolithography and wet etching. The etchant used was“ITO-07N” manufactured by Kanto Chemical Co., Inc. In this example, thewet etchability of the thin oxide films used in the experiment wasevaluated by optical microscopic observation. The evaluation resultsshow that residue did not occur due to wet etching for all thecompositions used in the experiment, and all the thin oxide films wereappropriately etched.

After the patterning, pre-annealing treatment was performed on the oxidesemiconductor films to improve the film quality thereof. Thepre-annealing was performed at 350° C. under atmospheric pressure for 1hour.

Next, a source-drain electrode was formed by a lift-off method usingpure Ti. Specifically, after patterning was carried out using aphotoresist, a Ti thin film was formed by a DC sputtering method (filmthickness 100 nm). A method for forming the Ti thin film for asource-drain electrode is the same as that in the case of the gateelectrode described above. Next, an unnecessary photoresist was removedby dipping in acetone with an ultrasonic washing apparatus to give TFTwith a channel length of 10 μm and a channel width of 200 μm.

After the source-drain electrode was formed as described, a passivationlayer was formed to protect each oxide semiconductor layer. As thepassivation layer, a layered film (total film thickness 400 nm) of SiO₂(film thickness 200 nm) and SiN (film thickness 200 nm) was used. Theabove-mentioned SiO₂ and SiN were formed by a plasma CVD method using“PD-220NL” manufactured by SAMCO Inc. In this example, after plasmatreatment was carried out by N₂O gas, the SiO₂ film and the SiN filmwere successively formed. A mixed gas of N₂O and N₂ diluted SiH₄ wasused for the formation of the SiO₂ film and a mixed gas of N₂ dilutedSiH₄, N₂ and NH₃ was used for the formation of the SiN film. In bothcases, the film formation power was set to 100 W and the film formationtemperature was set to 150° C.

Next, a contact hole for probing for evaluating transistorcharacteristics was formed in the passivation layer by photolithographyand dry etching. Next, an ITO film (film thickness 80 nm) was formedusing a DC sputtering method in conditions as follows: carrier gas:mixed gas of argon gas and oxygen gas, film formation power: 200 W, andgas pressure: 5 mTorr, to produce a TFT shown in FIG. 1.

Each TFT obtained as described above was subjected to investigations asfollows.

(1) Measurement of Transistor Characteristics

The transistor characteristics (drain current-gate voltagecharacteristics, Id-Vg characteristics) were measured using asemiconductor parameter analyzer (“4156C” manufactured by AgilentTechnologies). The detailed measurement conditions were as follows. Inthis example, the ON-current (Ion) at Vg=20 V was read, and the passcriterion was that Ion ≧1×10⁻⁵ A.

Source voltage: 0V

Drain voltage: 10V

Gate voltage: −30 to 30V (measurement interval: 0.25V)

(2) Threshold Voltage (Vth)

The threshold voltage is roughly a value of gate voltage at the timewhen a transistor is shifted from OFF state (state where drain currentis low) to ON state (state where drain current is high). In thisexample, the voltage in the case where the drain current is over 1 nAbetween ON-current and OFF-current is defined as the threshold voltage,and the threshold voltage of each TFT was measured. In this example, thepass criterion was that Vth (absolute value) is 5V or less.

(3) S Value

The S value (SS value) was defined as the minimum value of the gatevoltage necessary for increasing the drain current by one digit. In thisexample, the pass criterion was that S value is 1 V/dec or less.

(4) Carrier Mobility (Electron Field-Effect Mobility)

The carrier mobility (electron field-effect mobility) was calculated asthe mobility in a saturation region according to the followingexpression. In this example, the pass criterion was that the saturationmobility of 5 cm²/Vs or higher.

$\begin{matrix}{I_{d} = {\frac{1}{2}\mu_{FE}C_{OX}\frac{W}{L}\left( {V_{gs} - V_{th}} \right)^{2}}} & \left\lbrack {{Math}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Cox: insulator layer capacitance

W: channel width

L: channel length

Vth: threshold voltage

(5) Evaluation of Stress Stability (Light Irradiation and Negative BiasApplication as Stress)

In this example, stress tests were carried out by irradiation of lightwhile applying negative bias to a gate electrode for simulation ofenvironments (stress) at the time of actual panel drive. The stresstests conditions were as follows. A light wavelength with about 400 nmwas selected which was close to the band gap of an oxide semiconductorand with which the transistor characteristics tend to be easilyfluctuated.

Gate voltage: −20V

Substrate temperature: 60° C.

Light stress

Wavelength: 400 nm

Illuminance (light intensity for irradiation to TFT): 0.1 μW/cm²

Light source: LED manufactured by OptoSupply Limited (light quantity wasadjusted by an ND filter)

Stress tests time: 3 hour

Specifically, the threshold voltages (Vth) before and after the stresstests were measured using the technique described above, and adifference therebetween (ΔVth) was determined. In this example, the passcriterion was that the threshold voltage shift (the absolute value ofΔVth) is 15 V or less.

These results are shown in Table 1.

Also, for a portion of the examples, the resulting drain current-gatevoltage characteristics (Id-Vg characteristics) before and after thestress tests are shown in FIG. 3 (all of Nos. 2, 6, and 8 in Table 1 areexamples of the present invention). In these figures, the results beforethe stress tests are indicated by a dashed line, and the results afterthe stress tests (after three hours from the application) are indicatedby a solid line.

Expression (1) Expression (2) Expression (3) Value Value Value on the onthe on the ratio to [In] + [Zn] + [Sn] right determi- right determi-right determi- saturation absence of light irradiation Δ Vth No. [In][Zn] [Sn] side nation side nation side nation mobility Ion (A) S(V/dec )Vth (V) (V) 1 0.02 0.40 0.40 0.10 ∘ −0.87 ∘ 0.23 ∘ 14 0.003 0.4 −2  −122 0.10 0.54 0.36 0.04 ∘ −0 .64 ∘ 0.34 ∘ 11 0.003 0.4 −2  −8  3 0.20 0.530.27 0.01 ∘ −0.50 ∘ 0.41 ∘ 13 0.003 0.4 −2  −7  4 0.20 0.64 0.16 −0.06 ∘−0.19 ∘ 0.56 ∘ 11 0.003 0.4 −2  −6  5 0.10 0.45 0.45 0.10 ∘ −0.87 ∘ 0.23∘ 13 0.003 0.4 −1  −9  6 0.30 0.42 0.28 0.04 ∘ −0.64 ∘ 0.34 ∘ 15 0.0040.3 −3  −13 7 0.15 0.60 0.25 −0.01 ∘ −0.40 ∘ 0.46 ∘ 11 0.003 0.4 −1  −5 8 0.30 0.53 0.18 −0.04 ∘ −0.20 ∘ 0.51 ∘ 15 0.004 0.3 −3  −11 9 0.50 0.250.25 0.10 ∘ −0.87 ∘ 0.23 x 15 0.004 0.3 −4  −17 10 0.60 0.24 0.16 0.04 ∘−0.64 ∘ 0.24 x 19 0.004 0.3 −7  −18 11 0.25 0.53 0.23 −0.01 ∘ −0.41 ∘0.45 ∘ 13 0.003 0.4 −1  −10 12 0.15 0.64 0.21 −0.04 ∘ −0.30 ∘ 0.51 ∘ 100.003 0.4 −1  −9  13 0.15 0.30 0.55 0.17 x −1.21 ∘ 0.07 x 18 0.005 0.9−12 — 14 0.10 0.85 0.05 −0.14 ∘ 0.14 x 0.72 ∘ 4 0.0006 0.7 −7  — * Inthe column for expression (1), “value” indicates the value on the rightside of expression (1) and “determination” indicates that “∘” is usedwhen the relationship represented by expression (1) is satisfied and “x”is used when it is unsatisfied. * In the column for expression (2),“value” indicates the value on the right side of expression (2) and“determination” indicates that “∘” is used when the relationshiprepresented by expression (2) is satisfied and “x” is used when it isunsatisfied. * In the column for expression (3), “value” indicates thevalue on the right side of expression (3) and “determination” indicatesthat “∘” is used when the relationship represented by expression (3) issatisfied and “x” is used when it is unsatisfied.

Nos. 1 to 8, 11, and 12 in Table 1 satisfy all expressions (1) to (3)defined in the present invention, and therefore, had excellent TFTcharacteristics including mobility, and had ΔVth which was reduced to apredetermined range, and therefore, had excellent stress stability.

FIG. 3 shows graphs indicating changes in the TFT characteristics beforeand after the stress tests of Nos. 2, 6, and 8. In all cases, therequirements of the present invention were satisfied, and therefore, thestress stability was satisfactory, and satisfactory switchingcharacteristics were obtained even after the stress tests.

In contrast to this, Nos. 9 and 10 in Table 1 are examples in which theIn ratio represented by expression (3) does not satisfy the requirementsof the present invention, and in which ΔVth increased and therefore thestress stability significantly decreased. Furthermore, No. 10 in Table 1has a larger In ratio than that of No. 9, and therefore, Vth alsoincreased in the absence of light irradiation.

Also, No. 13 in Table 1 is an example which does not satisfy expressions(1) and (3), resulting in an increase in the Sn ratio, and in which theVth value increased and the TFT characteristics decreased. The presentinvention is intended to simultaneously obtain both satisfactory TFTcharacteristics and satisfactory stress stability. TFTs havingunsatisfactory TFT characteristics are not suitable for use even iftheir stress stability is satisfactory. Therefore, for above example,the stress stability test was not conducted (“-” is shown in the ΔVth(V) column in Table 1, and the same applies to other examples below).

Also, No. 14 in Table 1 is an example which does not satisfy expression(2) and has a large Zn ratio, and in which the mobility sharplydecreased and the Vth value significantly increased. Therefore, thestress stability test was not conducted.

The above experiment results show that the use of the IZTO semiconductorhaving the composition ratio defined by the present invention canprovide higher stress stability and satisfactory TFT characteristicswhile maintaining the mobility as high as that of conventional ZTO.Also, the semiconductor was satisfactorily processed by wet etching.Therefore, it is inferred that the oxide of the present invention mayhave an amorphous structure.

Example 2

In this example, the densities of oxide films (film thickness 100 nm)formed by using an oxide with the composition corresponding to Nos. 3,6, and 11 in Table 1 and controlling the gas pressure at the time ofsputtering film formation to 1 mTorr, 3 mTorr, or 5 mTorr were measured,and the mobility and the change quantity (ΔVth) of threshold voltageafter the stress test (light irradiation+negative bias application) wereinvestigated for a TFT produced in the same manner as in Example 1described above. A method for measuring the film density is as follows.

(Measurement of Density of Oxide Film)

The density of the oxide film was measured by XRR (X-ray reflectivitymethod). The detailed measurement conditions were as follows.

Analysis apparatus: Horizontal type x-ray diffraction apparatus SmartLab manufactured by Rigaku Co., Ltd.

Target: Cu (beam source: Kα ray)

Target output power: 45 kV-200 mA

Production of measurement sample

A sample used was produced by forming a film (film thickness 100 nm) ofan oxide with each composition on a glass substrate in the followingsputtering conditions, and thereafter carrying out the same heattreatment as that for pre-annealing treatment simulating thepre-annealing treatment in the TFT production process of Example 1 asdescribed above.

Sputtering gas pressure: 1 mTorr, 3 mTorr, or 5 mTorr

Oxygen partial pressure: O₂/(Ar+O₂)=2%

Film formation power density: DC 2.55 W/cm²

Heat treatment: 350° C. for 1 hour under an air atmosphere

These results are shown in Table 2. In Table 2, the values ofexpressions (4) and (5) and the determination results are additionallyshown for the purpose of reference. For example, in the column forexpression (4) in Table 2, “value” indicates the value on the right sideof expression (4), and “determination” indicates that “circles” is usedwhen the relationship represented by expression (4) is satisfied and“crosses” is used when it is unsatisfied. The same applies to expression(5).

TABLE 2 Gas pressure Expression (3) Expression (4) Expression (5) at theValue Value Value time of film Δ Vth on the on the on the formationDensity Movility determi- right determi- right determi- right determi-No. Composition (m Torr) (g/cm³) (cm²/Vs) (V) nation In ratio sidenation side nation side nation 1 Same as 1 6.2 16.6 −1.3  ⋆ 0.20 0.40 ∘0.20 ∘ 0.04 x 2 No.3 in 3 6.0 14.5 −4.2  ⋆ 0.20 0.40 ∘ 0.20 ∘ 0.04 x 3Table 1 In0.20, 5 5.8 12.7 −7.4  ⊚ 0.20 0.40 ∘ 0.20 ∘ 0.04 x Zn0.53,Sn0.27 4 Same as 1 6.2 18.2 −6.0  ⋆ 0.30 0.34 ∘ 0.14 x −0.02 x 5 No.6 in3 5.9 16.7 −10.7 ∘ 0.30 0.34 ∘ 0.14 x −0.02 x 6 Table 1 In0.30, 5 5.815.2 −13.4 ∘ 0.30 0.34 ∘ 0.14 x −0.02 x Zn0.42, Sn0.28 7 Same as 1 6.217.4 −4.2  ⋆ 0.25 0.45 ∘ 0.25 ∘ 0.09 x 8 No.11 in 3 5.9 15.6 −6.9  ⊚0.25 0.45 ∘ 0.25 ∘ 0.09 x 9 Table 1 In0.25, 5 5.8 13.3 −10.3 ∘ 0.25 0.45∘ 0.25 ∘ 0.09 x Zn0.53, Sn0.23

Nos. 3, 6, and 9 in Table 1 are the same as 3, 6, and 11 in Table 1 asmentioned above, respectively.

According to Table 2, the oxides in Table 2 satisfying all requirementdefined by the present invention all showed a high density of 5.8 g/cm³or higher. Examining the case using No. 3 in Table 1 as for example, thefilm density at the time of a gas pressure of 5 mTorr (No. 3) was 5.8g/cm³, whereas the film density at the time of a gas pressure of 3 mTorr(No. 2) was 6.0 g/cm³ and the film density at the time of a gas pressureof 1 mTorr (No. 1) was 6.2 g/cm³, and as the gas pressure was lowered, ahigher density was obtained. Also, as the film density was increased,the mobility was improved and the shift quantity of threshold value (theabsolute value of ΔVth) by the stress test was also lowered.

According to the experimental results, it was found that the density ofthe oxide film was changed in accordance with the gas pressure at thetime of sputtering film formation, and if the gas pressure was lowered,the film density was increased and accordingly, the electronfield-effect mobility was increased significantly and the shift quantityof threshold voltage (the absolute value of ΔVth) in the stress test(light irradiation+negative bias stress) was decreased. That issupposedly attributed to that the disturbance of sputtered atoms(molecules) can be suppressed by lowering the gas pressure at the timeof sputtering film formation to lessen the defects in the film, and thusthe mobility and the electric conductivity are increased to improve theTFT stability.

Although Table 2 shows the results which were obtained when the oxidesof Nos. 3, 6, and 11 of Table 1 were used, the above-describedrelationship between the density of the oxide film and the mobility ofTFT characteristics or the amount of a threshold voltage change afterthe stress test was similarly observed for other oxides satisfying therequirements of the present invention (e.g., Nos. 1, 2, 4, 5, 7, 8, and12 in Table 1).

EXPLANATION OF REFERENCE NUMERALS

-   1 Substrate-   2 Gate electrode-   3 Gate insulator layer-   4 Oxide semiconductor layer-   5 Source-drain electrode-   6 Passivation layer (insulator layer)-   7 Contact hole-   8 Transparent conductive film

1. An oxide, comprising Zn, Sn, and In, wherein the content (atomic %)of metal elements contained in the oxide is defined as [Zn], [Sn], and[In], respectively, and the content satisfies below expressions (1) to(3):[In]/([In]+[Zn]+[Sn])≧−0.53×[Zn]/([Zn]+[Sn])+0.36  (1),[In]/([In]+[Zn]+[Sn])≧2.28×[Zn]/([Zn]+[Sn])−2.01  (2),[In]/([In]+[Zn]+[Sn])≦1.1×[Zn]/([Zn]+[Sn])−0.32  (3), and wherein [In]is 0.1 or higher.
 2. (canceled)
 3. A thin-film transistor comprising asemiconductor layer comprising the oxide of claim
 1. 4. (canceled) 5.The thin-film transistor of claim 3, wherein the density of thesemiconductor layer is 5.8 g/cm³ or higher.
 6. A sputtering target,comprising Zn, Sn, and In, wherein the content (atomic %) of metalelements contained in the sputtering target is defined as [Zn], [Sn],and [In], respectively, and the content satisfies expressions (1) to(3):[In]/([In]+[Zn]+[Sn])≧−0.53×[Zn]/([Zn]+[Sn])+0.36  (1),[In]/([In]+[Zn]+[Sn])≧2.28×[Zn]/([Zn]+[Sn])−2.01  (2),[In]/([In]+[Zn]+[Sn])≦1.1×[Zn]/([Zn]+[Sn])−0.32  (3), and wherein [In]is 0.1 or higher.
 7. (canceled)
 8. The thin-film transistor of claim 4,wherein the density of the semiconductor layer is 5.8 g/cm³ or higher.